The versatility of UV curing

The versatility of UV curing

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UV curing is a versatile technology that enables inks, coatings, adhesives, and extrusions to be set in line, in a small footprint, and at high speed while also producing performance properties superior to what is achievable with conventionally dried materials. This is because UV curing is not drying and is instead a chemical reaction initiated with ultraviolet light energy. A reaction of this nature creates strong bonds between molecules and drives the creation of long polymer chains. To the observer, UV curing instantly transforms liquid-like materials that are wet to the touch into cross-linked solid polymers that are fully dry to the touch. 

UV formulations are 100% solids, contain no liquid carriers that must be evaporated, and require no energy consuming thermal dryers. This means UV curing processes are more environmentally friendly and produce less waste. Once a web or sheet exits a UV curing station, it is immediately ready for further processing, converting, sheeting, slitting, rewinding, and shipping. This keeps work in progress goods out of inventory.

Finally, UV cured surfaces do not scratch, mar, or become damaged when passed through downstream manufacturing line components or finishing equipment. This reduces scrap and facilitates quicker lead times.

UV formulations are easily applied, cured in a fraction of a second, and produce highly desirable and robust functional and aesthetic properties in final products. This facilitates use of UV processes across a wide range of printing and coating applications. Examples are highlighted in Table 1.

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UV curing sources

UV curing systems include mercury vapour, light emitting diode (LED), and excimer lamps.

While each of these technologies emit ultraviolet energy, the mechanisms that generate UV energy as well as characteristics of the corresponding UV output are very different. Examples of production needs solved with each type of UV curing system are detailed in Table 2.

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Mercury vapour lamps

Mercury vapour lamps are a type of medium-pressure, gas discharge lamp in which a small amount of elemental mercury and a specific mix of inert gas are vaporised into a plasma within a sealed quartz tube. Plasma is a high-temperature ionized gas capable of conducting electricity. It is produced by applying an electrical voltage between two electrodes within an arc lamp. Once vaporised, mercury plasma generates broad-spectrum UV output that radiates 360° from the quartz tube.

Specially designed reflectors located behind the quartz tube are used to concentrate emitted UV energy onto webs, sheets, or parts. UV curing systems utilizing mercury vapour lamps are cooled with forced air and/or circulated liquid. Mercury vapour lamps have a short warm-up and cool-down period that occurs each time they are switched on or off. An image of several mercury arc lamps and a lamp head assembly is provided in Figure 1 (a).

UV LED lamps

LED lamps are solid-state electronics composed of numerous chips of thin, semi-conductive, crystalline materials electrically wired together in a single row or a combination of rows and columns. In the case of UV LEDs, precisely designed and fabricated gallium nitride (GaN) and aluminum, gallium, nitride (AlGaN) materials emit relatively narrow wavelength bands of ultraviolet energy when connected to a DC power source.

The emitted light has a peak output centered at 365, 385, 395, or 405 nm, is projected forward from each LED a full 180°, is quickly and easily turned on or off, and has full linear adjustment of power. UV LEDs do not contain elemental mercury.

Each discrete LED is referred to as a positive-negative junction (p-n junction).This means that one portion of the LED has a positive charge, referred to as the anode (+).The other portion has a negative charge, referred to as the cathode (-).When connected to a DC voltage power supply, free electrons in the n-type region cross over and fill vacancies in the p-type region. As electrons flow across the boundary, they transition to a state of lower energy.

The respective drop in energy is released from the semiconductor as a combination of UV photons and heat. Typically, up to 40% of the differential is emitted as photons with electrical inefficiencies converting the balance into waste heat that must be removed from the system.

An illustration of three LED modules integrated into a much longer linear array with numerous modules as well as a corresponding lamphead assemblyis provided in Figure 1 (b). Each of the purple squares in the graphic represents a single LED. Engineers package and power LEDs to emit the desired UV output for the intended market segment and application. Lamphead assemblies are then cooled with forced air or circulated liquid to maintain the necessary device operating temperate, remove waste heat, and ensure performance longevity.

Excimer lamps

Like mercury vapour lamps, excimer lamps are a type of gas discharge lamp. Unlike broadband mercury vapour lamps, excimer lamps emit narrow bands of ultraviolet energy without the use of elemental mercury. Excimer lamps are commonly available with ultraviolet outputs centered at 172, 222, 308, and 351 nm.172 nm excimer lamps fall between 100 and 200 nm which is known as vacuum UV (VUV).

The VUV band represents the shortest wavelengths of ultraviolet light. These wavelengths are absorbed by oxygen and, therefore, do not travel far through air. Ozone is also generated when wavelengths of 240 nm and less are absorbed by oxygen molecules.

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Photons of vacuum UV contain more energy than those of longer UV wavelengths butare only effective at curing the outermost surface of UV formulations. In fact, 172 nm wavelengths are completely absorbed within the first 10 to 200 nm of UV formulated chemistry. As a result, 172 nm excimer lamps must always be integrated in combination with mercury or LED systems to achieve full depth of cure. Because the surface is crosslinked separately from the rest of the formulation, excimer UV curing isuniquely effective at creating matte surfaces that are ant-reflective, anti-glare, and anti-fingerprint.

It is important to note that since vacuum UV wavelengths are absorbed by oxygen molecules and incredibly efficient at generating ozone, 172 nm excimer lamps are always operated in a nitrogen inerted environment.

Excimer lamps consist of a quartz tube which serves as a dielectric barrier. The tube is filled with rare gases capable of forming excimer or exciplex molecules. Different gases produce different excited molecules and determine which specific wavelengths are emitted by the lamp. A coiled electrode runs along the inside length of the quartz tube while ground electrodes run along the outside length. Voltages are pulsed into the lamp at high frequencies. This causes electrons to flow within the internal electrode and discharge across the gas mixture toward the external ground electrodes.

This scientific phenomenon is known as dielectric barrier discharge (DBD). As electrons travel through the gas, they interact with atoms and create energized or ionized species that produce excimer or exciplex molecules. Excimer and exciplex molecules have an incredibly short life. As they decompose from an excited state to a ground state, a narrow band of photons are emitted. Excimer lamps generate negligible heat and require little if any lamphead assembly cooling. An image of an excimer lamp and corresponding lampheadis provided in Figure 1 (c).

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UV curing is practical for inkjet, flexo, gravure, screen, offset, slot-die, Mayer-rod, roller, curtain, and spray among many other transfer and deposition methods. Mercury arc lamps are available in lengths up to 2.5 meters; LED UV lamps in lengths up to 1.7 meters; and excimer lamps in lengths up to 2.3 meters. GEW supplies complete systems and ancillary components configured to meet the needs of each installation.

Systems are installed on new OEM presses and manufacturing lines and also retrofitted to existing machines. To find out more about UV curing products and how they benefit converters, visit GEW at www.gewuv.es

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